Lubricant compositions stabilized with styrenated phenolic antioxidant

ABSTRACT

Compositions are disclosed that comprise a lubricant, at least a first antioxidant and an optional second antioxidant, the first antioxidant being a styrenated phenolic antioxidant and the optional second antioxidant a secondary diarylamine for synergistic antioxidant action. Also disclosed is a method of increasing the oxidation stability of lubricating oils comprising: adding thereto at least a first antioxidant and, optionally, a second antioxidant, the first antioxidant being a styrenated phenolic antioxidant and the optional second antioxidant a secondary diarylamine.

TECHNICAL FIELD

This disclosure relates to an improvement in oxidation stability of lubricating oils by using a styrenated phenolic antioxidant and an optional synergistic secondary diarylamine.

BACKGROUND OF THE DISCLOSURE

Hydrocarbon based lubricants, when exposed to heat and oxygen (air) that are ubiquitously present during their manufacture, transport, storage, or use, will oxidize over time. Uncontrolled oil oxidation produces harmful species, which eventually compromises the designated functions of the lubricant, decreases the service life, and, to a greater extent, damages the machinery it lubricates. One practical approach to the prevention of lubricant oxidation is the employment of a suitable antioxidant system comprising one or more active components.

Driven by escalating performance and environmental requirements for many classes of lubricant products, the industry is continuously looking for high performance antioxidants to work with modern lubricant formulations to achieve, among other things, increased oxidative stability and drain intervals, improved low temperature properties, and greater fuel economy. One notable change from the lubricant formulation point of view is the reduction in the use of zinc dialkyldithiophosphates (ZDDP). Over the past decades, ZDDP's have been an important class of lubricant additive for many types of lubricants owing to their superior cost-effectiveness in wear protection and oxidation inhibition, particularly through a synergistic action with primary antioxidants. However, the presence of zinc contributes to the formation of ash particulates and volatile phosphors, that after entering the exhaust stream, poison the NOx catalysis, thus shortening the useful life of a catalytic converter.

In view of the aforementioned shortcomings of the known zinc and phosphorus-comprising additives, efforts have been made to provide lubricating oil additives that contain neither zinc nor phosphorus or, at least, contain them in substantially reduced amounts. It would therefore be desirable to provide improved additives for stabilizing and/or inhibiting lubricating oils from oxidative, thermal, and/or light-induced degradation while reducing the content of zinc and phosphorous employed in the lubricating oils.

According to the present disclosure, we have discovered an effective phenolic antioxidant that offers superior antioxidancy on its own and possesses unique antioxidant synergy when properly used in combination with a secondary diarylamine antioxidant for lubricant base stocks and/or lubricant formulations, particularly in those containing a low level of ZDDP.

SUMMARY OF THE DISCLOSURE

It has now been discovered that a class of styrenated phenolic antioxidants offers superior antioxidancy and their mixtures with a secondary diarylamine exhibit synergistic effects and therefore are more effective than using either of the materials alone in inhibiting oxidation of lubricating oil compositions. In particular, the styrenated phenolics act synergistically with alkylated diarylamines to provide significant improvements in oxidation control. More particularly, the present disclosure is directed to a lubricating oil composition comprising: (A) one or more base oils comprising API (American Petroleum Institute) Group I, Group II, Group III, Group IV and synthetic lubricating base stocks of varying viscosity grades;

(B) at least a first antioxidant selected from one or more hindered phenolics or an isomer or isomeric mixture thereof having the following general formula:

wherein R₁ and R₂ are independent and comprise hydrogen or styryl groups represented by the following formula (IA):

wherein n is an integer of from 0 to 5 and the α-position on the styryl group is optionally substituted with a hydrocarbyl group having from 1 to about 8 carbon atoms, and R₃ is a hydrogen or hydrocarbyl group having from 1 to about 100 carbon atoms; and (C) optionally, at least one second antioxidant comprising one or more secondary diarylamines having the following general formula:

(R₄)_(a)—Ar₁—NH—Ar₂—(R₅)_(b)   (II)

wherein Ar₁ and Ar₂ are independent and comprise aromatic hydrocarbons, R₄ and R₅ are independent and comprise hydrogen and hydrocarbyl groups and a and b are independent and 0 to 3, with the proviso that (a+b) is not greater than 4.

In another aspect, the present disclosure is directed to a method of increasing the oxidation stability of a lubricating oil, the method comprising: adding to a lubricating oil at least a first antioxidant comprising one or more hindered phenolic antioxidants represented by the general formula (I) and optionally, a second antioxidant comprising one or more secondary diarylamines represented by the general formula (II).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The term “hydrocarbyl” as used herein includes hydrocarbon as well as substantially hydrocarbon groups. “Substantially hydrocarbon” describes groups that contain heteroatom substituents that do not alter the predominantly hydrocarbon nature of the group. Examples of hydrocarbyl groups can include, but are not limited to, hydrocarbon substituents, substituted hydrocarbon substituents, and heteroatom substituents. Hydrocarbyl groups that may be utilized may contain from 1 to about 100 carbon atoms, preferably from about 6 to about 30 carbon atoms.

Hydrocarbon substituents can include, but are not limited to aliphatic, such as alkyl or alkenyl; alicyclic, such as cycloalkyl and cycloalkenyl; aromatic substituents; aromatic-, aliphatic-, and alicyclic-substituted aromatic substituents, and the like; as well as cyclic substituents wherein the ring is completed through another portion of the molecule (that is, for example, any two indicated substituents may together form an alicyclic radical).

Substituted hydrocarbon substituents, comprising those substituents containing non-hydrocarbon portions which, in the context of this disclosure, do not alter the predominantly hydrocarbon nature of the substituent, those skilled in the art will be aware of such groups that can include, halo, hydroxy, mercapto, nitro, nitroso, sulfoxy, and cyano, for example.

Heteroatom substituents, for example, substituents that will, while having a predominantly hydrocarbon character within the context of this disclosure, contain at least one atom other than carbon present in a ring or chain otherwise composed of carbon atoms, such as alkoxy or alkylthio, for example. Suitable heteroatoms will be apparent to those of ordinary skill in the art and can include, for example, sulfur, oxygen, nitrogen. In addition to heteroatoms, substituents containing heteroatoms can be included, such as, pyridyl, furyl, thienyl, and imidazolyl, for example. Preferably, no more than about 2, more preferably no more than one, substituent containing a heteroatom will be present for every ten carbon atoms in the hydrocarbyl group. Most preferably, there will be no such heteroatom substituents in the hydrocarbyl group.

As stated above, the hindered phenolic for use as the first antioxidant in the practice of this disclosure can be represented by the following formula (I):

wherein R₁ and R₂ are independent and hydrogen or styryl groups represented by the following formula (IA):

wherein n is an integer of from 0 to 5 and the α-position on the styryl group is optionally substituted with a hydrocarbyl group having from 1 to about 8 carbon atoms; and R₃ is a hydrogen or hydrocarbyl group having from one to about 100 carbon atoms, preferably from one to about 40 carbon atoms. The optional second antioxidant comprising one or more secondary diarylamines can be represented as having the following general formula:

(R₄)_(a)—Ar₁—NH—Ar₂—(R₅)_(b)   (II)

wherein Ar₁ and Ar₂ are independent and comprise aromatic hydrocarbons, and R₄ and R₅ are independent and comprise hydrogen and hydrocarbyl groups, and a and b are independent and 0 to 3, with the proviso that (a+b) is not greater than 4. The preferred aryl moieties suitable for the secondary diarylamine as represented by the general formula (II) are phenyl or naphthyl. There is no particular restriction on the type and total number of carbon atoms in the hydrocarbyl group, R₃ to R₅, of the hindered phenolics and the substituted secondary diarylamines as represented by the general formulae (I)-(II), respectively, with the proviso that the total number of carbon atoms render sufficient thermal stabilities and solubility of the additives in the base oil (A). Preferably, the hydrocarbyl moieties are alkyl, alkenyl, cycloalkyl, cycloalkenyl, aryl, arylalkyl, arylalkenyl, naphthyl, and naphthyl moieties that can be optionally substituted with alkyl, alkenyl, hydroxyl, and/or carboxyl groups, for example. The following are examples of preferred hydrocarbyls suitable for the practice of this disclosure: (a) straight chain branched chain alkyl or alkenyl groups containing one to 40 carbon atoms, even more preferably straight chain or branched chain alkyl groups containing one to 20 carbon atoms such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, 2-ethyl hexyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, oleyl, nonadecyl, eicosyl, isomers and mixtures thereof and the like; (b) cyclic alkyl and alkenyl groups such as cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclododecyl, cyclopentenyl, cyclohexenyl, cycloheptenyl, cyclooctenyl, cyclododecenyl, cyclopentadienyl, cyclohexadienyl, cycloheptadienyl, cyclooctadienyl, and the like with optionally substituted with one or more alkyl or alkenyl radicals having one to 40 carbon atoms, and more preferably one to 16 carbon atoms such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, isomers and mixtures of the foregoing, and the like; (c) phenyl, phenyl substituted with one or more alkyl or alkenyl radicals having one to 40 carbon atoms, and even more preferably one to 16 carbon atoms such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadeyl, hexadecyl, isomers of the foregoing, and the like; (d) naphthyl and naphthyl substituted with one or more alkyl or alkenyl radicals having one to 40 carbon atoms, and even more preferably one to 16 carbon atoms such as methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadeyl, hexadecyl, isomers and mixtures thereof of the foregoing, and the like; (e) heteroatom substituents, particularly alkoxyalkyl, alkoxyaryl groups having from one to 40 carbon atoms, and more preferably from one to 20 carbon atoms such as methoxymethyl, ethoxymethyl, ethoxyethyl, propoxymethyl, propoxyethyl, propoxypropyl, and the like; and phenyl substituted with one or more alkoxy groups having one to 16 carbon atoms, such as methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy, heptoxy, octoxy, nonoxy, decoxy, tridecyl, tetradecyl, pentadeyl, hexadecyl isomers and mixtures of the foregoing, and the like; (f) substituted hydrocarbon substituents, particularly hydroxyl, carboxyl, nitro, or cyano, for example.

With wide variation in the composition of the hydrocarbyl moieties, the hindered phenolic antioxidants represented by the formula (I) that are useful in the practice of this disclosure may include 2,6-bis(alpha-methylbenzyl)-4-methylphenol, 2,6-bis(alpha-methylbenzyl)-4-ethylphenol, 2,6-bis(alpha-methylbenzyl)-4-isobutylphenol,and the like; 2-alpha-methylbenzyl)-4-methylphenol, 2-alpha-methylbenzyl-4-ethylphenol, 2-alpha-methylbenzyl)-4-isobutylphenol, and the like; 2,6-bis(alpha-methylstyryl)-4-methylphenol, 2,6-bis(alpha-methylstyryl)-4-ethylphenol, 2,6-bis(alpha-methylstyryl)-4-isobutylphenol, and the like; 2-alpha-methylstyryl-4-methylphenol, 2-alpha-methylstyryl-4-ethylphenol, 2-alpha-methylstyryl-4-isobutylphenol), and the like; 2,2′-thiobis(6-alpha-methylbenzyl-4-methylphenol), 4,4′-thiobis(2,6-di-alpha-methylbenzyl-4-methylphenol), and the like; 2,2′-thiobis(6-alpha-methylstyryl-4-methylphenol), 4,4′-thiobis(2,6-di-alpha-methylstyryl-4-methylphenol), and the like; 2,2′-methylenebis(6-alpha-methylbenzyl-4-methylphenol), 4,4′-methylenebis(2,6-di-alpha-methylbenzyl-4-methylphenol), and the like; 2,2′-methylenebis(6-alpha-methylstyryl-4-methylphenol), and 4,4′-methylenebis(2,6-di-alpha-methylstyryl-4-methylphenol), for example.

According to an embodiment, the secondary diarylamines represented by the general formulae (II) that are useful in the practice of the present disclosure can include diphenylamine, monalkylated diphenylamine, dialkylated diphenylamine, trialkylated diphenylamine, and/or mixtures thereof, 3-hydroxydiphenylamine, 4-hydroxydiphenylamine, mono- and/or di-butyldiphenylamine, mono- and/or di-octyldiphenylamine, mono- and/or di-nonyldiphenylamine, phenyl-α-naphthylamine, phenyl-β-naphthylamine, diheptyldiphenylamine, mono- and/or di-(α-methylstyryl)diphenylamine, mono- and/or distyryidiphenylamine, 4-(p-toluenesulfonamido)diphenylamine, 4-isopropoxydiphenylamine, t-octylated N-phenyl-1-naphthylamine, mixtures of mono- and dialkylated t-butyl-t-octyldiphenylamines. The following are examples of preferred secondary diarylamines that are commercially available from the Ciba Corporation: Irganox® L67, Irganox L57, and Irganox L06. The following are examples of more preferred secondary diarylamines and are commercially available from the Chemtura Corporation: Naugalube® 438, Naugalube 438L, Naugalube 690, Naugalube 640, Naugalube 635, Naugalube 680, Naugalube AMS, Naugalube APAN, and Naugard PANA.

In the preparation of the lubricating oil compositions comprising component (B), the hindered phenolic antioxidant of the above general formulae (I), and the component (C), the secondary diarylamine with the above general formula (II), the antioxidants can be blended in the compositions in a range of about 0.01 to about 10 weight percent each, and preferably from about 0.1 to about 5 weight percent. The content ratio of the hindered phenolic antioxidant to the secondary diarylamine employed in the lubricating oil compositions of the present disclosure can be in practically all proportions. In illustrative embodiments, the ratio will be in the range of 1:99 to 99:1 parts by weight and more preferably, 90:10 to 10:90 parts by weight. The components (B) and (C) of the present disclosure can be pre-mixed according to the content ratio just defined then added to, or can be separately added to the lubricating oil (A) with the aid of mild heating (up to about 50° C.) and mechanical agitation as needed.

The antioxidants and the antioxidant mixtures of the present disclosure can be used in combination with other additives typically found in lubricating oils, as well as other antioxidants. The additives typically found in lubricating oils are, for example, dispersants, detergents, antiwear agents, antioxidants, friction modifiers, seal swell agents, demulsifiers, VI (viscosity index) improvers, pour point depressants, antifoamants, corrosion inhibitors, and metal deactivators. Such additives are well known to those skilled in the art and there is no particular restriction on the type of these additives for this disclosure. U.S. Pat. No. 5,498,809, incorporated herein by reference in its entirety, discloses useful lubricating oil composition additives.

Examples of dispersants can include polyisobutylene succinimides, polyisobutylene succinate esters, and Mannich Base ashless dispersants. Examples of detergents can include metallic and ashless alkyl phenates, metallic and ashless sulfurized alkyl phenates, metallic and ashless alkyl sulfonates, metallic and ashless alkyl salicylates, metallic and ashless saligenin derivatives.

Examples of antioxidants that can be used in combination with the antioxidant mixtures of the present disclosure can include dimethyl quinolines, trimethyldihydroquinolines and oligomeric compositions derived therefrom, thiopropionates, metallic dithiocarbamates, oil soluble copper compounds, and para-phenylenediamines. The following are examples of preferred substituted para-phenylenediamines that are commercially available from the Flexsys Corporation: Santoflex® IPPD, Santoflex 6PPD, Santoflex 44PD, Santoflex 77PD, Santoflex 134PD, Santoflex 1350PD, Santoflex 715PD, and Santoflex 434PD. An example of a more preferred substituted para-phenylenediamine is Naugalube® 403 which is commercially available from Chemtura Corporation.

Examples of anti-wear additives that can be used in combination with the additives of the present disclosure can include organoborates, organophosphites, organophosphates, organic sulfur-containing compounds, sulfurized olefins, sulfurized fatty acid derivatives (esters), chlorinated paraffins, zinc dialkyldithiophosphates, zinc diaryidithiophosphates, dialkyldithiophosphate esters, diaryl dithiophosphate esters, and phosphosulfurized hydrocarbons. The following are examples of the aforementioned additives and are commercially available from the Lubrizol Corporation: Lubrizol® 677A, Lubrizol 1095, Lubrizol 1097, Lubrizol 1360, Lubrizol 1395, Lubrizol 5139, and Lubrizol 5604, among others; and from the Ciba Corporation: Irgalube® 62, Irgalube 211, Irgalube 232, Irgalube 349, Irgalube 353, Irgalube TPPT, Irgafos® OPH, among others; and from the Chemtura Corporation: Durad® 40, Durad 48, Durad 60, Durad 125, Durad 220X, Durad 110, Durad 150, Durad 220, Durad 300, Durad 150B, Durad 220B, Durad 620B, Durad 310M, Weston® 600, Weston DLP, and Naugalube® TPP.

Examples of friction modifiers can include fatty acid esters and amides, organo molybdenum compounds, molybdenum dialkyldithiocarbamates, molybdenum dialkyl dithiophosphates, molybdenum disulfide, tri-molybdenum cluster dialkyldithiocarbamates, and non-sulfur molybdenum compounds, for example. The following are examples of molybdenum additives and are commercially available from the R. T. Vanderbilt Company, Inc.: Molyvan® A, Molyvan L, Molyvan 807, Molyvan 856B, Molyvan 822, and Molyvan 855, among others. In addition to the aforementioned friction modifiers, the following are also examples of such additives and are commercially available from the Asahi Denka Kogyo K.K.: SAKURA-LUBE® 100, SAKURA-LUBE 165, SAKURA-LUBE 300, SAKURA-LUBE 310G, SAKURA-LUBE 321, SAKURA-LUBE 474, SAKURA-LUBE 600, and SAKURA-LUBE 700, among others. The following are also examples of friction modifiers and are commercially available from Akzo Nobel Chemicals GmbH: Ketjen-Ox® 77M and Ketjen-Ox 77TS, among others. Naugalube® MolyFM is also an example and is commercially available from the Chemtura Corporation.

Examples of VI improvers can include olefin copolymers and dispersant olefin copolymers, for example. In certain embodiments, polymethacrylate and equivalents thereof are examples of pour point depressants. In another embodiment, polysiloxane and equivalents thereof can be employed as antifoamants. Examples of rust inhibitors can include polyoxyalkylene polyols, benzotriazole derivatives, and equivalents thereof. Examples of metal deactivators can include triazole, benzotriazole, 2-mercaptobenzothiazole, 2,5-dimercaptothiadiazole, tolyltriazole derivatives, and N,N′-disalicylidene-1,2-diaminopropane. The following are examples of metal deactivators and are commercially available from Ciba Corporation: Irgamet® 30, Irgamet 39 and Irgamet 42.

Lubricant Compositions

Compositions, when they contain the aforementioned additives, are typically blended into at least one base oil in amounts effective to provide their normal attendant functions. Representative effective amounts of such additives are illustrated in Table 1.

TABLE 1 Lubricant Composition More Preferred Additives Preferred Weight % Weight % V.I. Improver    1-12  1-4 Corrosion Inhibitor 0.01-3 0.01-1.5 Antioxidant 0.01-5 0.01-1.5 Dispersant  0.1-10 0.1-5  Lube Oil Flow 0.01-2 0.01-1.5 Improver Detergent/Rust 0.01-6 0.01-3   Inhibitor Pour Point   0.01-1.5 0.01-0.5 Depressant Anti-foaming Agents  0.001-0.1 0.001-0.01 Anti-wear Agents 0.001-5  0.001-1.5  Seal Swell Agents  0.1-8 0.1-4  Friction Modifiers 0.01-3 0.01-1.5 Lubricating Base Oil Balance Balance

Additional additives can be employed and in such cases it may be desirable, although not necessary, to prepare additive concentrates. Additive concentrates can comprise concentrated solutions or dispersions of the subject additives of this disclosure in amounts described above. These concentrates can comprise more than one additive and as such can be referred to as an additive-package. As an additive-package, several additives can be added simultaneously to the base oil to form the lubricating oil composition. Dissolution of the additive concentrate into the lubricating oil can be facilitated by solvents and/or by mixing accompanied by mild heating. The concentrate or additive-package can be formulated to contain the additives in proper amounts to provide the desired concentration in the final formulation when the additive-package is combined with a predetermined amount of base oil. Thus, the subject additives of the present disclosure can be added to small amounts of base oil and/or compatible solvents along with other desirable additives to form additive-packages. These additive packages can contain active ingredients in collective amounts of from about 2.5 to about 90 percent, preferably from about 15 to about 75 percent, and more preferably from about 25 percent to about 60 percent by weight additives in the appropriate proportions with the remainder being base oil. The final formulations can employ from about 1 to 20 weight percent of the additive-package with the remainder comprising base oil. Preferably, the total phosphorus content in the final formulations will be less than about 600 ppm.

All of the weight percentages expressed herein (unless otherwise indicated) are based on the active ingredient (AI) content of the additive, and/or upon the total weight of any additive-package, or formulation, which will be the sum of the AI weight of each additive plus the weight of total oil or diluent.

In general, the additives of the present disclosure are useful in a variety of lubricating oil base stocks. The lubricating oil base stock is any natural or synthetic lubricating oil base stock fraction having a kinematic viscosity at 100° C. of about 2 to about 200 cSt, more preferably about 3 to about 150 cSt, and most preferably about 3 to about 100 cSt. The lubricating oil base stock can be derived from natural lubricating oils, synthetic lubricating oils, or mixtures thereof. Suitable lubricating oil base stocks include base stocks obtained by isomerization of synthetic wax and wax, as well as hydrocracked base stocks produced by hydrocracking (rather than solvent extracting) the aromatic and polar components of the crude oil. Natural lubricating oils include animal oils, such as lard oil, tallow oil, vegetable oils including canola oils, castor oils, and sunflower oils, for example, petroleum oils, mineral oils, and oils derived from coal or shale.

Synthetic oils include hydrocarbon oils and halo-substituted hydrocarbon oils, such as polymerized and interpolymerized olefins, gas-to-liquids prepared by Fischer-Tropsch technology, alkylbenzenes, polyphenyls, alkylated diphenyl ethers, alkylated diphenyl sulfides, as well as their derivatives, analogs, homologs, and the like. Synthetic lubricating oils also include alkylene oxide polymers, interpolymers, copolymers, and derivatives thereof, wherein the terminal hydroxyl groups have been modified by esterification, and etherification, for example.

Another suitable class of synthetic lubricating oils comprises the esters of dicarboxylic acids with a variety of alcohols. Esters useful as synthetic oils also include those made from C₅ to C₁₈ monocarboxylic acids and polyols and polyol ethers. Other esters useful as synthetic oils include those made from copolymers of α-olefins and dicarboxylic acids which are esterified with short or medium chain length alcohols.

Silicon-based oils, such as the polyalkyl-, polyaryl-, polyalkoxy-, or polyaryloxy-siloxane oils and silicate oils, comprise another useful class of synthetic lubricating oils. Other synthetic lubricating oils include liquid esters of phosphorus-containing acids, polymeric tetrahydrofurans, poly α-olefins, and the like.

The lubricating oil may be derived from unrefined, refined, re-refined oils, or mixtures thereof. Unrefined oils are obtained directly from a natural source or synthetic source (e.g., coal, shale, or tar and bitumen) without further purification or treatment. Examples of unrefined oils include a shale oil obtained directly from a retorting operation, a petroleum oil obtained directly from distillation, or an ester oil obtained directly from an esterification process, each of which is then used without further treatment. Refined oils are similar to unrefined oils, except that refined oils have been treated in one or more purification steps to improve one or more properties. Suitable purification techniques can include distillation, hydrotreating, dewaxing, solvent extraction, acid or base extraction, filtration, percolation, and the like, all of which are well-known to those skilled in the art. Re-refined oils are obtained by treating refined oils in processes similar to those used to obtain the refined oils. These re-refined oils are also known as reclaimed or reprocessed oils and often are additionally processed by techniques for removal of spent additives and oil breakdown products.

Lubricating oil base stocks derived from the hydroisomerization of wax may also be used, either alone or in combination with the aforesaid natural and/or synthetic base stocks. Such wax isomerate oil is produced by the hydroisomerization of natural or synthetic waxes or mixtures thereof over a hydroisomerization catalyst. Natural waxes are typically the slack waxes recovered by the solvent dewaxing of mineral oils; synthetic waxes are typically the wax produced by the Fischer-Tropsch process. The resulting isomerate product is typically subjected to solvent dewaxing and fractionation to recover various fractions having a specific viscosity range. Wax isomerate is also characterized by possessing very high viscosity indices, generally having a VI of at least 130, preferably at least 135 or higher and, following dewaxing, a pour point of about −20° C. or lower.

The lubricating oil used in the practice of the present disclosure can be selected from any of the base oils in Groups I-V as broadly specified in the American Petroleum Institute (API) Base Oil Interchangeability Guidelines. The five base oil groups are described in Table 2.

TABLE 2 API Base Oil Category Saturates Category Sulfur (%) (%) Viscosity Index Group I >0.03 and/or <90 80 to 120 Group II ≦0.03 and ≧90 80 to 120 Group III ≦0.03 and ≧90 ≧120 Group IV All polyalphaolefins (PAOs) Group V All others not included in Groups I, II, III or IV

The additives of the present disclosure are especially useful as components in many different lubricating oil compositions. The additives can be included in a variety of oils with lubricating viscosity, including natural and synthetic lubricating oils and mixtures thereof. The additives can be included in crankcase lubricating oils for spark-ignited and compression-ignited internal combustion engines. The compositions can also be used in gas engine lubricants, steam and gas turbine lubricants, automatic transmission fluids, gear lubricants, compressor lubricants, metal-working lubricants, hydraulic fluids, and other lubricating oil and grease compositions.

The advantages and the important features of the present disclosure will be demonstrated in the following examples.

EXAMPLE 1

The antioxidant efficacy of the hindered phenolic and the synergistic effects; from a combined usage of the hindered phenolic and the secondary diarylamine, have been demonstrated in a low-phosphorus 5W20 engine oil formulation by using the pressurized differential scanning calorimetry (PDSC) test and the Mid-High Temperature Thermo-oxidation Engine Oil Simulation Test (TEOST, MHT).

Preparation of a Low-Phosphorus 5W20 Engine Oil Formulation

The low-phosphorous 5W20 engine formulation was pre-blended using the following commercially available compositions. There is no particular restriction on the type and exact composition of the materials in the context of the present disclosure.

TABLE 3 Low-phosphorus 5W20 Engine Oil Pre-blend Composition Amounts, wt. % Base oil, API Group II Balance Overbased Calcium sulfonate 2.5 detergents Zinc dialkyldithiophosphate 0.5 Succinimide dispersant 6.5 Pour Point Depressant 0.1 VI improver 5.0

To the 5W20 engine oil pre-blend set forth in Table 3 was added antioxidants, as depicted in Tables 5 and 7 below, in preparation of 5W20 engine oils. The finished engine oils contain approximately 450 ppm of phosphorus derived from (ZDDP).

Pressurized Differential Scanning Calorimetry Results

The Pressurized Differential Scanning Calorimetry (PDSC) measures the oxidation induction time (OIT) of oil. The instrument used is a Mettler® DSC27HP manufactured by Mettler-Toledo, Inc (Switzerland). The instrument has a typical repeatability of ±2.5 minutes with 95 percent confidence over an OIT of 100 minutes. The PDSC test conditions are given in Table 4. For every 50 grams of test blend prepared, 40 μl of oil soluble ferric naphthenate (6 weight percent in mineral oil) was added, prior to PDSC testing, to facilitate 50 ppm of iron in oil. At the beginning of a PDSC run, the steel cell is pressurized with oxygen and heated at a rate of 40° C. per minute to the prescribed isothermal temperature. The induction time is measured from the time the sample reaches its isothermal temperature until the enthalpy change is observed. The longer the oxidation induction time, the better the oxidation stability of the oil.

TABLE 4 PDSC Test Conditions Test Parameters Settings Isothermal Temperature 160° C. or 185° C. Cell Pressure 500 psi of O₂ O₂ Gas Flow Rate Through Cell 100 ml/min. Catalyst 50 ppm of Iron Sample Holder Open Aluminum Pan Sample size 1.0-2.0 mg Induction Time Enthalpy Change

The finished 5W20 engine oils were tested in the PDSC using the conditions set forth in Table 4. The test results in terms of average OIT and standard deviation of each duplicate run are given in Table 5 below. The exceedingly long OIT results of blends 1 and 3 in the data table as compared to the respective comparatives demonstrate that the engine oil compositions containing the hindered phenolic antioxidant of the present disclosure have superior oxidative stabilities.

TABLE 5 PDSC Results of the 5W20 Engine Oil Blends Containing a Single Antioxidant. PDSC Test T OIT, SD Blend Objective Antioxidant Wt. % (° C.) (min.) (min.) 1 Disclosure STP 0.5 160 52.9 1.3 Composition 2 Comparison HPE 0.5 160 18.6 1.6 3 Disclosure STP 1.5 185 10.7 1.2 Composition 4 Comparison HPE 1.5 185 2.2 0.2 STP = 2,6-bis(alpha-methylbenzyl)-4-methylphenol. HPE = 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid, C₇-C₉ branched alkyl esters.

Mid-High Temperature Thermo-oxidative Engine Oil Simulation Test

The Mid-High Temperature Thermo-oxidative Engine Oil Simulation Test (MHT TEOST) was performed according to the ASTM D7097 standard procedure to determine the deposit forming tendencies of the engine oil. TEOST determines the mass of deposit formed on a specially constructed steel rod by continuously stressing a repetitive passage of 8.5 ml of test oil under thermal-oxidative and catalytic conditions for 24 hours. The less the amount of deposits obtained, the better the oxidation stability of the oil. Throughout the 24 hour test duration, volatile compounds of the test oil that are there originally or formed because of the oxidation of the oil are also collected in a small vial by means of condensation. Table 6 summarizes the test conditions.

TABLE 6 TEOST MHT Test Conditions (ASTM D 7097) Test Parameters Settings Test duration 24 hours Rod Temperature 285° C. Sample size 8.5 g (mixture of 8.4 g of oil and 0.1 g of catalyst) Sample flow rate 0.25 g/min Flow rate (dry air) 10 mL/min Catalyst Oil soluble mixture containing Fe, Pb, and Sn

The results obtained from the TEOST testing of the finished 5W20 engine oils are given in Table 7. Based on the data, the improved deposit control efficacy of the additives of this disclosure has been clearly demonstrated. The significantly lower amounts of deposits obtained for blends 6 and 7 as compared to that of the blend 5, the pre-blend, and compared to the comparative blends 8 and 9 respectively containing an equal level of a commercial hindered phenolic ester antioxidant demonstrate that the lubricating oil compositions containing the antioxidant of this disclosure have superior oxidative stability to better control deposit formation in the TEOST.

TABLE 7 TEOST Results of the 5W20 Engine Oil Blends Containing a Single Antioxidant Total deposits, Volatiles, Blend Objective Antioxidant Wt. % mg mg 5 5W20 Pre- Nil — 132 4238 blend 6 Disclosure STP 1.0 77 4081 Composition 7 Disclosure STP 1.5 51 2706 Composition 8 Comparison HPE 1.0 82 4507 9 Comparison HPE 1.5 64 3322 STP = 2,6-bis(alpha-methylbenzyl)-4-methylphenol. HPE = 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid, C₇-C₉ branched alkyl esters.

In the TEOST, the synergistic antioxidant effect from a proper mixing of the styrenated phenolics and a secondary diarylamine according to the practice of this disclosure in stabilizing the 5W20 engine oil formulation is more clearly seen. As indicated by the data tabulated in Table 8, the lower amounts of deposits obtained for blends 10 and 11 demonstrate that the lubricating oils containing appropriate mixtures of the antioxidant mixtures according to the present disclosure have superior oxidative stability to better control deposit formation in the TEOST.

TABLE 8 TEOST Results of the 5W20 Engine Oil Blends Containing Multiple Antioxidants TEOST De- Antioxidant Antioxidant posits, Volatiles, Blend Objective 1, (wt. %) 2, (wt. %) mg mg 10 Disclosure STP (0.25) NDPA (0.75) 47 2917 Composition 11 Disclosure STP (0.20) NDPA (0.80) 45 2763 Composition 12 Comparison — NDPA (1.00) 55 3287 13 Comparison HPE (0.20) NDPA (0.80) 42 2951 14 Comparison SP (0.20) NDPA (0.80) 49 3192 STP = 2,6-bis(alpha-methylbenzyl)-4-methylphenol. HPE = 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid, C₇-C₉ branched alkyl esters. NDPA = nonylated diphenylamine. SP = 2,2′-thiobis(4-methyl-6-butyl-phenol)

EXAMPLE 2

The antioxidant efficacy of the hindered phenolic and the synergistic effects from a combined usage of the hindered phenolic and the secondary diarylamine according to the practice of this disclosure have been demonstrated in an industrial turbine oil formulation, tested by using the Rotating Pressure Vessel Oxidation Test (RPVOT) method.

Preparation of an Industrial Turbine Formulation

An industrial turbine oil pre-blend was first prepared with the following commercially available components. There is no particular restriction on the type and exact composition of the materials in the context of the present disclosure.

TABLE 9 Turbine Oil Pre-blend Component Wt. % Base oil, API Group II Balance Corrosion Inhibitor 0.05 Metal Deactivator 0.03 Defoamer 0.005

To the turbine oil pre-blend as set forth in Table 9 was added an antioxidant as depicted in Table 11 in preparation of a variety of fully formulated industrial turbine oils each containing 1.0 percent by weight of antioxidants in total.

Rotating Pressure Vessel Oxidation Test (RPVOT)

The Rotating Pressure Vessel Oxidation Test (RPVOT) was conducted according to the standard test method specified by ASTM D 2272. The RPVOT utilizes an oxygen-pressured vessel to evaluate the oxidation stability of new and in service turbine oils, having the same composition (base stock and additives), in the presence of water and a copper catalyst coil at 150° C. In a vessel equipped with a pressure gauge, the test oil, water, and a copper catalyst coil, which are separately contained in a covered glass container, are placed. The vessel is charged with oxygen to a pressure of 90 psi and placed in a constant temperature oil bath set at 150° C., and rotated axially at 100 rpm at an angle of 30 degrees from the horizontal. The number of minutes required to reach a specific drop 25 psi in gage pressure indicates the oxidation stability of the test sample. The longer the duration to reach the required pressure drop, the better oxidative stability of the test sample. Table 10 lists the RPVOT test conditions.

TABLE 10 RPVOT Test Conditions Initial Conditions Copper Catalyst Coil Weight 55.6 grams Sample Size Weight 50.00 grams Distilled Water weight 5 grams Temperature, C. 150° C. Oxygen Initial Pressure at RT 90° C. Oxygen Max Pressure at 150 C. 188 psi Pressure Drop to End Test 25 psi

The RPVOT test results of the turbine oils are set forth in Table 11. It can be seen from the above data that the turbine oil formulation containing the styrenated phenolic antioxidant (blend 17) of the present disclosure possessed better oxidative stability than the turbine oil pre-blend (blend 15) and the comparative blend 20 containing an equal amount of hindered phenolic ester. It can also be seen from the above data that the turbine oil formulation containing a proper mixture of the styrenated phenolic antioxidant and the secondary diarylamine (blend 19) according to the practice of the presentation disclosure exhibited a synergy that better stabilized the turbine oil.

TABLE 11 RPVOT Results of the Turbine Oils Antioxidant Antioxidant RPVOT SD Blend Objective 1, (wt. %) 2, (wt. %) (min.) (min.) 15 Turbine oil — — 24 1 pre-blend 16 Comparison — NDPA 1144 6 (1.00) 17 Disclosure STP (1.00) — 338 1 Composition 19 Disclosure STP (0.25) NDPA 1331 13 Composition (0.75) 20 Comparison HPE (1.00) — 229 0 21 Comparison HPE (0.25) NDPA 1231 11 (0.75) STP = 2,6-bis(alpha-methylbenzyl)-4-methylphenol. HPE = 3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid, C₇-C₉ branched alkyl esters. NDPA = nonylated diphenylamine (Naugalube 438L).

In view of the many changes and modifications that can be made without departing from principles underlying the present disclosure, reference should be made to the appended claims for an understanding of the scope of the protection to be afforded the disclosure. 

1. A composition comprising: (a) at least one lubricating oil comprising one or more Group I, Group II, Group III, Group IV or synthetic lubricating base stocks of varying viscosity grades; (b) at least a first antioxidant comprising one or more hindered phenolics having the general formula:

or an isomer or isomeric mixture thereof wherein R₁ and R₂ are independent and hydrogen or styryl groups represented by formula (IA):

wherein n is an integer of from 0 to 5 and the α-position on the styryl group is optionally substituted with a hydrocarbyl group having from 1 to about 8 carbon atoms; and R₃ is hydrogen or a hydrocarbyl group; and (c) an optional second antioxidant comprising one or more secondary diarylamines having the general formula: (R₄)_(a)—Ar₁—NH—Ar₂—(R₅)_(b) wherein Ar₁ and Ar₂ are independent and comprise one or more aromatic hydrocarbons, and R₄ and R₅ are independent and comprise one or more hydrogen or hydrocarbyl groups, and a and b are independent and integers from 0 to 3, with the proviso that (a+b) is not greater than
 4. 2. A composition of claim 1 further comprising at least one additional additive comprising one or more dispersants, detergents, rust inhibitors, metal deactivators, antiwear agents, antifoamants, friction modifiers, seal swell agents, demulsifiers, viscosity index improvers, and pour point depressants.
 3. A composition of claim 1 wherein the lubricant is suitable for use in a high temperature and iron catalyzed environment.
 4. A composition of claim 3 wherein the lubricant is a grease.
 5. A method of increasing the oxidation stability of a lubricating oil comprising one or more Group I, Group II, Group III, Group IV or synthetic lubricating base stocks of varying viscosity grades comprising adding thereto a composition comprising from about 0.01 to about 10 weight percent of a first antioxidant and from about 0.01 to about 10 weight percent of a second antioxidant, the first antioxidant comprising one or more hindered phenolics having the general formula:

or an isomer or isomeric mixture thereof wherein R₁ and R₂ are independent and hydrogen or styryl groups represented by formula (IA):

wherein n is an integer of from 0 to 5 and the α-position on the styryl group is optionally substituted with a hydrocarbyl group having from 1 to about 8 carbon atoms; and R₃ is hydrogen or a hydrocarbyl group and the second antioxidant comprising one or more secondary diarylamines having the general formula: (R₄)_(a)—Ar₁—NH—Ar₂—(R₅)_(b) wherein Ar₁ and Ar₂ are independent and comprise one or more aromatic hydrocarbons, and R₄ and R₅ are independent and comprise one or more hydrogen or hydrocarbyl groups, and a and b are independent and integers from 0 to 3, with the proviso that (a+b) is not greater than
 4. 6. The method of claim 5 wherein the content ratio of the first antioxidant to the second antioxidant is from 1:99 to 99:1.
 7. The method of claim 5 further comprising one or more dispersants, detergents, rust inhibitors, metal deactivators, antiwear agents, antifoamants, friction modifiers, seal swell agents, demulsifiers, viscosity index improvers, and pour point depressants.
 8. A method of claim 5 wherein the composition comprises about 600 ppm or less of phosphorus. 